Re: TEST: 6 of 7: empty universe example,

"owl:Thing owl:equivalentClass owl:Nothing" is *inconsistent* in OWL-DL.

In OWL-DL (as in FOL), the universe of the interpretation is always
required to be a non-empty set. If there is no non-empty interpretations
of an ontology, then the ontology is unsatisfiable (and hence
inconsistent in proof-theoretic terms).
As owl:Nothing is the complement of owl:Thing, you can reformulate the
above axiom as:
owl:Thing owl:equivalentClass complementOf(owl:Thing) which is obviously
inconsistent in DL and FOL, being an example of "A <=> not A".

Mehrdad Omidvari